A bowler's success is, to a very great extent, dependent upon maintaining the optimum body position at the critical point at which the foul line is approached and the arm swings forward with the ball. The upper body should be precisely perpendicular to the bowler's target line (a straight line extending away from the bowler along which the ball begins its movement down the lane) so that the throwing arm can swing freely at a ninety degree angle to the shoulder axis. Bowlers who do not achieve this "square" body position may be able to execute effective shots on occasion, but the consistency required to become a bowler of the first rank will allude them.
Maintenance of a proper upper body position is made difficult by the required foot movement toward the end of the bowler's approach. The last step is taken with the foot on the opposite side from the ball. Thus a right-handed bowler steps last with the left foot which moves forward with a sliding motion as the ball is released. Accordingly, in the case of a right-handed bowler, the left foot is referred to as the "sliding foot" and the right foot as the "non-sliding foot." High quality bowling shoes provide a left shoe with a sole or a part of a sole at the toe that slides more easily than the right shoe. The sole of the left shoe may be smooth leather, while the sole of the right shoe is rougher leather or rubber (this being reversed for a left-handed bowler). Any rubber used is white rubber to avoid marking the lane.
The sliding foot should ideally be placed under the center of gravity of the bowler's body and should slide toward the pins while aligned with the direction in which the ball is thrown. This can be a difficult motion and many bowlers incorrectly place the sliding foot on the lane at such an angle that it is pointed toward the side on which the ball is carried. As a result, the upper body tends to depart from the desired square position and the entire delivery is adversely affected. The follow-through angle becomes less than ninety degrees, as explained in more detail below, with a loss of effectiveness when the ball contacts the pins.